Polypeptide growth factors are mitogens that act on cells by specifically binding to receptors located on the cell plasma membrane. The platelet-derived growth factor (PDGF) stimulates a diverse group of biochemical responses, e.g., changes in ion fluxes, activation of various kinases, alteration of cell shape, transcription of various genes, and modulation of enzymatic activities associated with phospholipid metabolism. See, e.g., Bell et al. (1989) "Effects of Platelet Factors on Migration of Cultured Bovine Aortic Endothelial and Smooth Muscle Cells," Circulation Research 65:1057-1065.
Platelet-derived growth factors are found in higher animals, particularly in warm blooded animals, e.g., mammals. In vitro, PDGF is a major polypeptide mitogen in serum for cells of mesenchymal origin such as fibroblasts, smooth muscle cells, and glial cells. In vivo, PDGF does not normally circulate freely in blood, but is stored in the alpha granules of circulating blood platelets. During blood clotting and platelet adhesion the granules are released, often at sites of injured blood vessels, thereby implicating PDGF in the repair of blood vessels. PDGF may stimulate migration of arterial smooth muscle cells from the medial to the intimal layer of the artery where the muscle cells may proliferate. This is likely to be an early response to injury.
PDGF has also been implicated in wound healing, in atherosclerosis, in myeloproliferative disease, and in stimulating genes associated with cancerous transformation of cells, particularly c-myc and c-fos.
The platelet-derived growth factor is composed of two homologous polypeptide chains; it is a dimer of 16 kilodalton proteins which are disulfide connected. These polypeptides are of two types, the type B chain and the type A chain. Three forms of the growth factor dimer are found corresponding to a homodimer of two type A chains, a homodimer of two type B chains, and a heterodimer of the type A chain with the type B chain. Each of these three different combinations is referred to as a PDGF isoform. See, for a review on PDGF, Ross et al. (1986) "The Biology of Platelet-Derived Growth Factor," Cell 46:155-169. The growth factor sequences from mouse and human are highly homologous.
The PDGF acts by binding to the platelet-derived growth factor receptor (PDGF-R). The receptor is typically found on cells of mesenchymal origin. The functional receptor acts while in a form comprising of two transmembrane glycoproteins, each of which is about 180 kilodaltons. Two different polypeptides have been isolated, a type B receptor polypeptide and a type A receptor polypeptide.
A sequence of a type B receptor polypeptide of the mouse platelet-derived growth factor receptor polypeptide is published in Yarden et al. (1986) Nature 323:226-232. A sequence of an type A human platelet-derived growth factor receptor (hPDGF-R) polypeptide is disclosed in Matsui et al. (1989) Science 243:800-803.
These PDGF receptors usually have three major identifiable regions. The first is a transmembrane region (TM) which spans the plasma membrane once, separating the regions of the receptor exterior to the cell from the regions interior to the cell. The second region is an extracellular region (XR) which contains the domains that bind the polypeptide growth factor (i.e., the ligand binding domains). The third is an intracellular region (IR) which possesses a tyrosine kinase activity. This tyrosine kinase domain is notable in having an insert of about 100 amino acids, as compared with most other receptor tyrosine kinase domains which are contiguous or have shorter insert segments.
The complete sequences of the human type B and human type A receptor polypeptides are reported elsewhere, e.g., U.S. Ser. No. 07/309,322, which is hereby incorporated herein by reference. However, for many purposes, a smaller or less than full length functional protein would be desired. For example, smaller molecules may be more easily targeted to areas of compromised circulation, or present fewer epitopes or extraneous domains unrelated to various activities of interest. Functional analogues with a slightly modified spectrum of activity, or different specificity would be very useful.
Thus, the use of new composite constructs exhibiting biological activity in common with platelet-derived growth factor receptor polypeptides will have substantial use as research reagents, diagnostic reagents, and therapeutic reagents. In particular, the identification of important polypeptide features in the extracellular region of the platelet-derived growth factor receptor polypeptides will allow substitutions and deletions of particular features of the domains. Moreover, use of an in vitro assay system provides the ability to test cytotoxic or membrane disruptive compounds.